Home / IB DP Math AA Topic : AHL 3.12 :Unit vectors; base vectors i, j, k. HL Paper 2

# IB DP Math AA Topic : AHL 3.12 :Unit vectors; base vectors i, j, k. HL Paper 2

## Question

Two submarines A and B have their routes planned so that their positions at time t hours, 0 ≤ t < 20 , would be defined by the position vectors rA and relative to a fixed point on the surface of the ocean (all lengths are in kilometres).

To avoid the collision submarine B adjusts its velocity so that its position vector is now given by

a. Show that the two submarines would collide at a point P and write down the coordinates of P.[4]

b.i.Show that submarine B travels in the same direction as originally planned.[1]

b.ii. Find the value of t when submarine B passes through P.[2]

c.i. Find an expression for the distance between the two submarines in terms of t.[5]

c.ii. Find the value of t when the two submarines are closest together.[2]

c.iii. Find the distance between the two submarines at this time.[1]

## Markscheme

rA = rB (M1)

2 − t = − 0.5t ⇒ t = 4 A1

checking t = 4 satisfies 4 + t = 3.2 + 1.2t and − 1 − 0.15t = − 2 + 0.1t R1

P(−2, 8, −1.6) A1

Note: Do not award final A1 if answer given as column vector.

[4 marks]

Note: Accept rArB.

distance $$D = \sqrt {{{\left( {2 – 0.55t} \right)}^2} + {{\left( {0.8 – 0.08t} \right)}^2} + {{\left( {1 – 0.24t} \right)}^2}}$$ M1A1

$$\left( { = \sqrt {8.64 – 2.688t + 0.317{t^2}} } \right)$$

[5 marks]

c.i.

minimum when $$\frac{{{\text{d}}D}}{{{\text{d}}t}} = 0$$ (M1)

t = 3.83 A1

[2 marks]

c.ii.

0.511 (km) A1

[1 mark]

c.iii.
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